Exim verify=header_syntax buffer overflow

Background

Exim is an highly configurable message transfer agent (MTA) developed at the University of Cambridge.

Description

When the option “verify = header_syntax” is used in an ACL in the configuration file, Exim is vulnerable to a buffer overflow attack that can be triggered remotely by sending malicious headers in an email message. Note that this option is not enabled in Exim’s default configuration file.

Impact

This vulnerability can be exploited to trigger a denial of service attack and potentially execute arbitrary code with the rights of the user used by the Exim daemon (by default this is the “mail” user in Gentoo Linux).

Workaround

Make sure the verify=header_syntax option is not used in your exim.conf file.

Resolution

All users of Exim should upgrade to the latest stable version:

 # emerge sync

 # emerge -pv ">=mail-mta/exim-4.33-r1"
 # emerge ">=mail-mta/exim-4.33-r1"